A variety of promoters are used in constructs for gene expression. The choice of promoter will often be influenced by the specific use that the construct is being employed for. However, in general constructs that provide high-level expression over a sustained period are desired, particularly for therapeutic applications, but also where it is desired to express genes to harvest the expressed proteins and in instances such as agriculture to obtain desired characteristics in reared animals.
In the use of constructs in a therapeutic context sustained expression at a high level is particularly important. Achieving sustained and high-level expression may mean that a particular treatment has to be given less often and remains effective for longer. In chronic conditions and inherited genetic defects this may be particularly important where in essence the underlying defect means that continuous treatment has to be given. Examples of such conditions include Cystic Fibrosis where treatment may have to be given permanently and hence any means of increasing the interval between treatments is important.
Gene expression constructs can suffer from a variety of problems. In some cases expression may be only for a short period before being silenced. This is particularly the case in vivo and in a variety of tissues. Additionally, or alternatively, some constructs give rise to only very weak expression and inadequate expression to achieve the desired effect.
A further problem with some constructs for gene expression when employed in vivo is that they may trigger the immune system of the subject in an undesired way. Thus, a subject may display an immune response against particular viral gene expression constructs that limit their effectiveness, particularly when used repeatedly in the same subject which may be the situation as outlined above for many conditions.